A typical gearbox includes a main housing having a hub, a rotary shaft mounted to the hub via a bushing, a worm wheel mounted in the main housing and fixed to the rotary shaft, an output gear located outside the main housing and fixed to the rotary shaft. Once the worm wheel is driven, a rubber gasket, a driving plate, the rotary shaft and the output gear are driven to rotate coaxially. The worm wheel, rubber gasket, driving plate, rotary shaft, and output gear typically all have complicated structures and are connected in a complex manner.